This invention relates to a novel branched siloxane and also to silicone based release coating compositions and release modifier compositions containing the aforementioned branched siloxane.
Silicone based release coatings are useful in applications where relatively non-adhesive surfaces are required. Single sided liners, for example, backing papers for pressure sensitive adhesive labels, are usually adapted to temporarily retain the labels without affecting the adhesive properties of the labels. Double sided liners for example, interleaving papers for double sided and transfer tapes, are utilised to ensure the protection and desired unwind characteristics of a double sided self-adhesive tape or adhesive film.
A substrate, for example a single sided liner, is coated by applying a silicone based release coating composition onto the substrate and subsequently curing the composition, by, for example, thermally initiated hydrosilylation.
The basic constituents of silicone based release coating compositions which are cured by hydrosilylation are:
1) an alkenylated polydiorganosiloxane, typically a linear polymer with terminal alkenyl groups,
2) a polyorganohydrogensiloxane cross-linking agent, designed to cross-link the alkenylated polydiorganosiloxane and
3) a catalyst, to catalyse the aforementioned cross-linking reaction.
Often a fourth constituent, an inhibitor designed to prevent the commencement of curing below a prerequisite cure temperature, is also included in the composition.
Silicone based release coating compositions consisting of the three essential constituents and optionally the inhibitor are generally referred to as premium release coating compositions.
In order to control the level of release force from a release coating it has become common practice for a silicone based release coating composition to contain an additive, generally known as a release modifier. The release modifier usually replaces a proportion of the alkenylated polydiorganosiloxane in a premium release coating composition.
Improvements in the performance of release coatings are continuously being sought with respect to, for example, ease of cure, i.e. the decrease in cure times at relatively low temperatures, anchorage of coatings to a substrate and release performance. One factor which particularly necessitates continued development of release coatings is the use of an ever increasing number of substrates, for example, polypropylene, polyethylene and polyester onto which release coating compositions are applied and cured.
U.S. Pat. No. 4,772,515 describes a silicone release coating composition having the three essential constituents discussed above, wherein the polydiorganosiloxane must have in a single molecule at least two groups of the following general formula:
xe2x80x83Rc(Rf)2SiO1/2((Rg)2SiO2/2)hRjSiO3/2
where Rc is an alkenyl group; each Rf, Rg and Rj are independently hydrogen, hydroxyl or a monovalent organic group and h is 0 or an integer between 1 and 300 inclusive. The resultant release coating composition is made by merely mixing the three constituents together.
U.S. Pat. No. 4,774,111 describes a silicone based release coating composition comprising an organosilicon compound having on average one to three silicon bonded monovalent hydrocarbon or halohydrocarbon radicals per silicon atom. An average of at least two of these groups being olefinic groups and the remaining silicon vacancies being satisfied by divalent radicals free of aliphatic unsaturation which link the silicon atoms together.
EP-A-559575 describes a silicone based release composition containing the three essential constituents and an additional constituent comprising a silicone containing a plurality of ethylenically unsaturated groups having from four to twelve carbon atoms wherein from 25 to 90 mole % of the total number of siloxane groups are T units.
For the sake of clarification M, D, T and Q units represent units of the empirical formulae R3SiO1/2, R2SiO2/2, RSiO3/2 and SiO4/2 respectively, wherein each R group is a monovalent substituent.
U.S. Pat. No. 5,616,672 describes the use of a substantially branched curable alkenyl silicone consisting of M, D and T groups and having on average at least two branching points per molecule. These compounds are said to be useful in release coating compositions.
U.S. Pat. No. 5,063,254 describes a polysiloxane comprising a linear polydiorganosiloxane chain having 25 to 1000 repeating units joined at one end to an acrylic end group and at its other end to a crosslinked MTQ moiety wherein the ratio of M units to T and Q units M/(T+Q) is 0.55-0.75. The M and/or T units may also contain (meth)acrylate groups. The polysiloxane has utility in potting and conformal coating.
A branched siloxane according to a first aspect of the present invention consists of:
i) one or more Q units of the formula(SiO4/2) and
ii) from 15 to 995 D units of the formula Rb2SiO2/2 which units (i) and (ii) may be inter-linked in any appropriate combination, and
iii) M units of the formula RaRb2SiO1/2, wherein each Ra substituent is selected from the group consisting of an alkyl group having from 1 to 6 carbon atoms, an alkenyl group having from 1 to 6 carbon atoms and an alkynyl group having from 1 to 6 carbon atoms, at least three Ra substituents in the branched siloxane being alkenyl or alkynyl units, and each Rb substituent is selected from the group consisting of an alkyl group having from 1 to 6 carbon atoms, an alkenyl group having 2 to 6 carbon atoms, an aryl group, an alkoxy group, an acrylate group and a methacrylate group.
The branched siloxane preferably contains at least two polydiorganosiloxane chains of the formula (Rb2SiO2/2)n where each n is independently from 2 to 100.
A branched siloxane according to a second aspect of the invention contains at least three aliphatically unsaturated hydrocarbon groups, terminated by units of the formula RaRb2SiO1/2 and otherwise consists of:
i) one or more units of the formula(SiO4/2); and
ii) at least two polydiorganosiloxane chains of the formula (Rb2SiO2/2)n, where each n is independently from 2 to 100, the total Rb2SiO2/2 units in the branched siloxane being from 15 to 995 units, wherein each Ra substituent is selected from the group consisting of an alkyl group having from 1 to 6 carbon atoms, an alkenyl group having from 1 to 6 carbon atoms and an alkynyl group having from 1 to 6 carbon atoms and each Rb substituent is selected from the group consisting of an alkyl group having from 1 to 6 carbon atoms, an aryl group, an alkoxy group, an acrylate group and a methacrylate group.
Preferably the aliphatically unsaturated hydrocarbon groups are either alkenyl or alkynyl groups.
Preferably at least 50% of Ra substituents are alkenyl groups. Most preferably each Ra substituent is an alkenyl group.
Each alkenyl group may be selected from vinyl, allyl, butenyl, pentenyl and hexenyl groups but is preferably selected from a vinyl (vi) and a hexenyl (hex) group.
The branched siloxane most preferably comprises at least one Q unit bonded to four (Rb2SiO2/2)n chains and for example can have the formula 
where each n is independently from 1 to 100.
Preferably each Rb substituent is an alkyl groups for example, a methyl, ethyl, propyl, isopropyl, butyl, pentyl or hexyl group; preferred are methyl and ethyl groups but the most preferred is a methyl group. Hence, in the case when there is only a single unit of the formula SiO4/2 present in the branched siloxane according to the first aspect of the present invention, the branched siloxane may have substantially the following formula wherein each n is independently from 1 to 100 when each Ra substituent is a vinyl group and each Rb group is a methyl group. 
It is to be appreciated that in view of the size of the branched siloxane, it is possible that a very small number of the siloxane units present in the branched siloxane (preferably less than 1%) may be units of the formula RbSiO3/2, where Rb is as previously defined, may occur.
Preferably the branched siloxane in accordance with the first aspect of the present invention has a viscosity of not less than 50 mm2/s and not more than 10000 mm2/s at 25xc2x0 C., more preferably the viscosity is from 50 to 1000 mm2/s.
Henceforth the number of siloxane units in the branched siloxane will be referred to as the degree of polymerisation (DP).
The DP of the branched siloxane may be from 20 to 1000. A preferred range is from 20 to 500 but the most preferred range is from 20 to 250.
A method according to the invention for the preparation of a branched siloxane comprises the steps of:
a) mixing a compound having the general formula (SiO4/2)(RaRb2SiO1/2)4 with a cyclic polydiorganosiloxane, and/or a substantially linear hydroxy terminated polydiorganosiloxane wherein each Ra substituent is selected from the group consisting of an alkyl group having from 1 to 6 carbon atoms, an alkenyl group having from 1 to 6 carbon atoms and an alkynyl group having from 1 to 6 carbon atoms and each Rb substituent is selected from the group consisting of an alkyl group having from 1 to 6 carbon atoms, an aryl group, an alkoxy group, an acrylate group and a methacrylate group;
b) causing the mixture to react in the presence of an acid or phosphazene base catalyst at a temperature of up to 180xc2x0 C.; and
c) neutralising the reaction mixture
Preferably each cyclic polydiorganosiloxane contains from 3 to 10 Rb2SiO2/2 units although it is preferred that the cyclic polydiorganosiloxanes are polydialkylsiloxane rings consisting of from 3 to 6 repeating Rb2SiO2/2 units in which each Rb substituent is a methyl group.
The type of reaction which takes place is either an acid or a base catalysed equilibration reaction dependent on the chosen catalyst.
In the case of acid catalysed equilibration reactions, the acid catalyst used may be any catalyst suitable the catalysis of an acid based equilibration reaction for example trifluoromethyl sulphonic acid, acid clays, for example, ambelyst and phosphonitrile chloride catalysts. A preferred catalyst is a trifluoromethane sulphonic acid.
In the case of basic catalysed equilibration preparations the catalyst may be any suitable phosphazene base catalyst, for example phosphazene bases of the following general formulae:
((R12N)3Pxe2x95x90Nxe2x80x94)k(R12N)3xe2x88x92kPxe2x95x90NR2
[((R12N)3Pxe2x95x90Nxe2x80x94)k(R12N)3xe2x88x92kPxe2x80x94N(H)R2]+[Axe2x88x92]
or
[(R12N)3Pxe2x95x90Nxe2x80x94)l(R12N)4xe2x88x92lP]+[A]xe2x88x92
in which R1, which may be the same or different in each position, is hydrogen or an optionally substituted hydrocarbon group, preferably a C1-C4 alkyl group, or in which two R1 groups bonded to the same N atom may be linked to complete a heterocyclic ring, preferably a 5- or 6-membered ring; R2 is hydrogen or an optionally substituted hydrocarbon group, preferably a C1-C20 alkyl group, more preferably a C1-C10 alkyl group; k is 1, 2 or 3, preferably 2 or 3; 1 is 1, 2, 3 or 4, preferably 2, 3 or 4; and A is an anion, preferably fluoride, hydroxide, silanolate, alkoxide, carbonate or bicarbonate. Particularly preferred are aminophosphazenium hydroxides.
In the case of the acid catalysed equilibration reaction the reaction mixture is preferably maintained at a temperature of from 75xc2x0 to 120xc2x0, most preferably the reaction mixture is maintained at a temperature of from 80 to 90xc2x0 C. in the presence of a water co-catalyst. In the case of the base catalysed equilibration reaction the reaction mixture is preferably maintained at a temperature of from 120xc2x0 and 160xc2x0, most preferably the reaction mixture is maintained at a temperature of from 130xc2x0 to 150xc2x0 C.
Any appropriate neutralising agent may be utilised the choice clearly being dependent on the acidic or basic nature of the catalyst, examples include the use of bisdimethylsilylphosphonate for base catalysed equilibrium reactions and calcium carbonate for acid catalysed equilibration reactions.
The amount of each constituent used in the method is dependent on two factors, the required degree of polymerisation of the branched siloxane and the number of alkenyl groups required in the branched siloxane. Preferably there is from 1.1 to 22.1% by weight of (SiO4/2)(RaRb2SiO1/2)4 in the mixture of step (a), more preferably from 2.21 to 11.04% by weight of (SiO4/2)(RaRb2SiO1/2)4 present and most preferably from 3.45 to 6.9% by weight. The remainder is made up to 100% by weight with the cyclic polydiorganosiloxane and/or the substantially linear hydroxy terminated polydiorganosiloxane.
A release coating composition according to the present invention comprises:
1) a branched siloxane as described above;
2) an organohydrogenpolysiloxane cross-linking agent in an amount such that the ratio of the total number of Sixe2x80x94H groups in the composition to aliphatically unsaturated hydrocarbon groups in the composition is from 0.9:1 to 3:1; and
3) a sufficient amount of a hydrosilylation catalyst effective to catalyse the reaction between the branched siloxane and the cross-linking agent.
The organohydrogenpolysiloxane cross linking agent must contain at least three Sixe2x80x94H groups and may have the general formula:
Rt3SiO1/2((CH3)2SiO2/2)d(Rt2SiO2/2)e)SiO1/2Rt3
where each Rt may be an alkyl group having 1 to 4 carbon atoms or hydrogen, d is 0 or an integer, e is an integer such that d+e is from 8 to 100. Alternatively the cross-linking agent may be an MQ resin consisting of units of the general formula SiO4/2 and Rq3SiO1/2 wherein at least one Rq substituent is a hydrogen atom and the remainder are alkyl groups.
Preferably the ratio of the total amount of Sixe2x80x94H groups:alkene groups in the release coating composition is in the range of from 1.1:1 to 2.5:1, most preferably the range is from 1.2:1 to 2:1.
Suitable hydrosilylation catalysts include complexes or compounds of group VIII metals, for example, platinum, ruthenium, rhodium, palladium, osmium and indium. Preferred catalysts are platinum compounds or complexes including chloroplatinic acid, platinum acetylacetonate, complexes of platinous halides with unsaturated compounds, for example, ethylene, propylene, organovinylsiloxanes and styrene, hexamethyldiplatinum, PtCl2. PtCl3 and Pt(CN)3. Alternatively the catalyst may be a rhodium complex, for example, RhCl3(Bu2S)3.
The composition may additionally comprise one or more inhibitors adapted to prevent the cure of the coating composition from occurring below a predetermined temperature. Whilst an inhibitor is not essential to the functioning of the coating composition itself it is to be understood that in the absence of an inhibitor the catalyst may initiate/catalyse the cure of the silicone based release coating composition at ambient temperature, once the three essential constituents have been mixed together.
Examples of suitable inhibitors include ethylenically or aromatically unsaturated amides, acetylenic compounds, ethylenically unsaturated isocyanates, olefinic siloxanes, unsaturated hydrocarbon diesters, conjugated eneynes, hydroperoxides, nitrites and diaziridines, specific examples include methyl butynol, dimethyl hexynol or ethynyl cyclohexanol, trimethyl(3,5-dimethyl-1-hexyn-3-oxy)silane, a maleate for example, Bis(2-methoxy-1-methylethyl)maleate, a fumarate e.g. diethylfumarate or a fumarate/alcohol mixture wherein the alcohol is, for example, benzyl alcohol or 1-octanol and ethenyl cyclohexyl-1-ol.
Preferably the release coating composition in accordance with the fourth aspect of the present invention has a viscosity of not less than 50 mm2/s and not more than 10000 mm2/s at 25xc2x0 C., more preferably the viscosity is from 50 to 1000 mm2/s so that the branched siloxane is of a suitable viscosity for coating a substrate. If the viscosity is lower than 50 mm2/s problems may occur with the wetting of a substrate surface by the release coating composition containing the branched siloxane. If the viscosity is higher than 10000 mm2/s then the release coating composition containing the branched siloxane is too viscous for use in the present application.
The release coating composition may additionally comprise a dialkyl alkenyl silyl terminated polydiorganosiloxane having a viscosity at 25xc2x0 C. of at least 50 mm2/s of which a dimethyl vinyl silyl terminated or dimethyl hexenyl silyl terminated polydimethylsiloxane is preferred.
Other constituents which may also be added to release coating compositions of the present invention include, for example, silicone release modifiers, fillers, reactive diluents, adhesion promoters, solvents, fragrances, preservatives and fillers, for example, silica, quartz and chalk.
Any appropriate silicone release modifier may be utilised, examples include for example one or more of an alkenylated silicone resin, an alkenylated polydiorganosiloxane, one or more primary alkenes containing from 12 to 30 carbon atoms, and one or more branched alkenes containing at least 10 carbon atoms.
Bath life extenders may be present in an amount sufficient to retard the curing reaction at room temperature. Examples may include compounds which contain one or more primary or secondary alcohol groups, for example, aliphatic and aromatic alcohols with fewer than 10 carbon atoms, such as methanol, ethanol, propanol phenol and cyclohexanol, carboxylic acids and cyclic ethers.
Furthermore, the composition may also comprise a linear alkenyl dialkyl silyl terminated polydiorganosiloxane.
The release coating compositions may be applied solventless or in a solvent or as part of an oil-in-water emulsion.
The present coating composition may be utilised for release purposes on a variety of substrates including paper and films. The films may for example be polyethylene, polypropylene, polyester, polystyrene, oriented polypropylene or biaxially oriented polypropylene films.
It is well known that release coating compositions which may be cured at low temperatures have a tendency to poor long term anchorage. It has been found that release coatings in accordance with the fourth aspect of the present invention both cure at relatively low temperatures and have improved long term anchorage properties. As the cure is good at low temperatures there is minimal transfer of silicones to an adhesive such as the adhesive on a label, which in turn provides the benefit that the strength of the adhesive is maintained. When high cure temperatures are required for release coatings problems occur with the quality of the substrate itself which cause shrinkage or brittle films which in turn means that significantly less rehumidification of the substrate is required after cure of the release coating.
While release coating compositions of the present invention may be prepared by merely premixing the three essential constituents together with any optional ingredients present it may be more desirable to prepare such compositions in separate parts or packages i.e. in the form of a kit. In such a case the portions are combined at the time the composition is to be applied as a coating. With respect to the essential ingredients, the kit may contain either:
a first part comprising the branched siloxane and inhibitor, a second part comprising a release modifier and inhibitor, a third part comprising the catalyst and a fourth part comprising the cross-linking agent; or
a first part comprising the branched siloxane and catalyst, a second part comprising a release modifier and the catalyst and a third part comprising the cross-linking agent and inhibitor.
A silicone based release modifier composition in accordance with the present invention may comprise a branched siloxane as described above and at least one additional component selected from the following components:
i) an alkenylated silicone resin
ii) an alkenylated polydiorganosiloxane, and
iii) one or more primary alkenes containing from 14 to 30 carbon atoms, and
iv) one or more branched alkenes containing at least 10 carbon atoms.
Such a release modifier composition is added to a silicone release coating composition.
The alkenylated silicone resin (i) is most preferably at least one alkenylated MQ resin wherein the M groups have the general formula R23SiO1/2 and are typically trialkyl siloxy and/or dialkyl alkenyl siloxy groups. The alkenyl group may be selected from the group consisting of cyclohexenyl, vinyl, propenyl, butenyl, pentenyl and hexenyl. Most preferably the alkenyl group is a vinyl or hexenyl group. The alkyl groups may be any suitable alkyl groups, but are most preferably methyl groups. The Q groups are groups of the formula SiO4/2 and these M and Q groups may be present in any appropriate ratio.
The alkenylated polydiorganosiloxane, is preferably an alkenyldialkyl silyl terminated polydiorganosiloxane comprising units of the formula R2SiO2/2 wherein each R group is an alkyl group having from 1 to 6 carbon atoms or one R group is an alkyl group as defined and one is an alkenyl group having from 1 to 6 carbon atoms, preferably a vinyl or hexenyl group.
The or each primary alkene (iii) may be any primary alkene containing from 10 to 30 carbon atoms such as, for example, tetradecene and octadecene. The or each branched alkene (iv) may be any one or more suitable branched alkenes, for example, one or more branched alkenes of the general formula 
wherein the n number of methylene groups and m number of branched alkyl groups are randomly distributed in the chain, n and m are independently 0 or an integer of from 1 to 20, x, z and each y is independently an integer of from 1 to 12. Preferably the total number of carbon atoms in each alkene is at least 20. It is to be noted that release modifiers containing substantially linear alkenes such as for example component (iii) above tend to cause smoking during the curing process of a coating. It has been found that replacement of component (iii) with a branched alkene as exemplified by component (iv) significantly reduces this problem.
Such a release modifier preferably comprises from 25 to 85% by weight of the branched siloxane in accordance with the present invention, the remainder being made up of one or more of components (i), (ii), (iii) or (iv).
One advantage in using a release modifier in accordance with the present invention is that by replacing at least a proportion of substantially linear alkene with the branched siloxane the smoking effect often seen during the cure process is significantly reduced. It has also been found that the level of extractables, i.e. the level of uncured siloxane, is maintained at the same level or is even reduced when compared to the level of extractables obtained from the unmodified release coating, whereas usually with prior art release modifiers the level of extractables obtained increases, sometimes significantly.
The release modifier composition as described above may be incorporated into a release coating composition in accordance with the present invention or may alternatively be incorporated in a release coating composition comprising an alkenylated polyorganosiloxane, an organohydrogenpolysiloxane cross-linking agent and an effective amount of a hydrosilylation catalyst.
In order that the invention may be become more clear there now follow a number of detailed examples.